2.A.6.2.2Multidrug/dye/detergent/bile salt/organic solvent resistance pump (substrates include: chloramphenicol, tetracycline, erythromycin, nalidixic acid, fusidic acid, fluoroquinolones, lipophilic β-lactams, norfloxacin, doxorubicin, novobiocin, rifampin, trimethoprim, acriflavin, crystal violet, ethidium, disinfectants, rhodamine-6G, TPP, benzalkonium, SDS, Triton X-100, deoxycholate/bile salts/organic solvents (alkanes), growth inhibitory steroid hormones (estradiola and progesterone), and phospholipids) (Elkins and Mullis, 2006). Lateral entry of substrates from the lipid bilayer into AcrB and its homologues has been proposed (Yu et al., 2003a; 2003b). [An asymmetric trimeric structure is established with AcrA having a hexameric structure, and TolC having a trimeric structure (Seeger et al., 2006]. A structure of a complex with YajC is also known (Törnroth-Horsefield et al., 2007). A covalently linked trimer of AcrB provides evidence for a peristaltic pump, alternative access, rotation mechanism (Takatsuka and Nikaido, 2009;Nikaido and Takatsuka, 2009; Pos, 2009) Further evidence for a rotatory mechanisms stems from kinetic analyses for cephalosporin efflux which can exhibit positive cooperativity (Nagano and Nikaido, 2009). May also export signaling molecules for cell-cell communication (Yang et al., 2006). The substrates may be captured in the lower cleft region of AcrB, then transported through the binding pocket, the gate, and finally to the AcrA funnel that connects AcrB to TolC (Husain & Nikaido et al., 2010). AcrB has been converted into a light-driven proton pump using delta-rhodopsin (dR) linked to AcrB via a glycophorin A transmembrane domain. This created a
solar powered protein capable of selectively capturing antibiotics from bulk solutions (Kapoor and Wendell 2013). The trimeric structure is essential for activity (Ye et al. 2014). Association with AcrZ (TC# 8.A.50), a small 1 TMS protein (49 aas) that modifies the substrate specificity of AcrAB, has been demonstrated (Hobbs et al. 2012). In a similar way, the binding of YajC to AcrB stimulates the export of ampicillin (Törnroth-Horsefield et al. 2007). AcrZ binds to
AcrB in a concave surface of the transmembrane domain (Du et al. 2015). Substrate binding accelerates conformational transitions and substrate dissociation, demonstrating cooperativity (Wang et al. 2015). The overall structure of AcrAB-TolC exemplifies
the adaptor bridging model, wherein the funnel-like AcrA hexamer forms an intermeshing cogwheel
interaction with the alpha-barrel tip region of TolC. Direct interaction between AcrB and TolC
is not allowed (Kim et al. 2015). TMS2 in AcrB is required for lipophilic carboxylate binding. A groove shaped by the interface between TMS1 and TMS2 specifically binds fusidic acid and other lipophilic
carboxylated drugs (Oswald et al. 2016). After ligand binding, a proton may bind to an acidic residue(s) in the transmembrane domain, i.e., Asp407 or Asp408, within the putative network of electrostatically interacting residues, which also include Lys940 and Thr978, and this may initiate a series of conformational changes that result in drug expulsion (Su et al. 2006). His978 is probably on the H+ pathway (Takatsuka and Nikaido 2006). AcrAB-TolC segregates to the old pole following cell division, causing the two daughter cells to exhibit different drug resistances (Bergmiller et al. 2017). The hoisting-loop is a highly flexible hinge that enables conformational energy transmission (Zwama et al. 2017). AcrB exhibits three distinct conformational states in the transport cycle, substrate access, binding, and extrusion, or loose (L), tight (T), and open (O) states, respectively (Yue et al. 2017). Simulations show that both Asp407 and Asp408 are deprotonated in the L/T states, while only Asp408 is protonated in the O state. Release of a proton from Asp408 in the O state results in large conformational changes. Simulations offer dynamic details of how proton release drives the O-to-L transition in AcrB (Yue et al. 2017). The three-dimensional structures of the homo-trimer complexes of AcrB-like transporters, and a three-step functional rotation helps to explain the mechanism of transport, but a more comprehensive model has been proposed (Zhang et al. 2017). Preparation of the trimeric complex (AcrAB/TolC) for cryo EM has been described (Du et al. 2018). The structural and energetic basis behind coupling functional rotation to proton translocation has been presented (Matsunaga et al. 2018). Protonation of transmembrane Asp408 in the drug-bound protomer drives rotation. The conformational pathway identifies vertical shear motions among several transmembrane helices, which regulate alternate access of water as well as peristaltic motions that pump drugs into the periplasm (Matsunaga et al. 2018). CryoEM of detergent-free AcrB preserves lipid-protein interactions for visualization and reveals how the lipids pack against the protein (Qiu et al. 2018).